。
I hear from mother from time to time.
Literal
Mother [from-から] sometimes word [subject-が] exists (polite).
便りがある is the fixed idiom for 'hear from someone' — literally 'there is word (from X).' The source is marked with から. This is an older, slightly more literary expression than the more modern 連絡がある; 便り specifically carries a 'letter/written communication' flavor, while 連絡 is more general.